Global View of the Nighttime low Latitude Ionosphere by the 135.6 nm OI Observation with IMAGE/FUV

The auroral camera onboard the IMAGE satellite (FUV) has been taking snapshots of Earth's atmosphere. The FUV/SI (Spectral Imager) instrument takes images at two wavelengths (121.8 nm (SI12) and 135.6 nm (SI13)). The SI13 measures 135.6 nm emissions, which is emitted by atomic oxygen, with a narrow spectral resolution of 8.0 nm. During nighttime at mid and low latitudes when emission due to particle excitation is negligible, the 135.6 nm emission originates in radiative recombination of oxygen ions and additionally from reactions involving negative oxygen ion (Dymond et. al., 1997). Therefore, the intensity of the 135.6 nm emission is a good indicator of ionospheric O+ ions integrated along the line of sight. Because the latitude of IMAGE's apogee drifts toward lower latitudes, FUV is now viewing the low latitude region of the earth. In particular, the SI13 image is clearly showing the nighttime "subtropical UV arcs" corresponding to the ionospheric equatorial anomaly (EA). The two minute cadence of the observations provides a unique data set of the low-latitude nightime ionosphere, though the instrument is more suited to making measurements of the bright aurora. The instrumental sensitivity limits the interpretation of ionospheric emissions to the area of the subtropical arcs. In this report, we present IMAGE observations of the low latitude ionosphere and compare it with the model calculation of the 135.6 nm emission rate based on the SAMI-2 ionosphere model (Huba et. al., 2000). Acknowledgements: This work uses the SAMI2 ionosphere model written and developed by the Naval Research Laboratory. Reference: Dymond, K.F., et. al., Radio Sci., 32, 1985, 1997 Huba, J.D., et. al., JGR, 105, 23,035, 2000.